10.14.2007 at 09:23 |  
In this section we give a brief overview of a procedural model forskeletal support. The model is implemented in AL, a proceduralmodeling and animation language with facilities for definingand manipulating articulated models. We introduce articulationvariables (or avars) to the model and use them to provide animationand interaction controls. The model is applied to the armskeleton to illustrate its operation. This example will be extendedin the next section when the modeling of muscles is considered.

Bones and joints

Since bones are hard relative to other anatomical structures in thehuman body, a rigid model for individual bones is appropriate. Wemodel bones with functions that select one representation out ofa number of alternatives based on a complexity parameter. Two ofthese alternatives, constructed in piecewise fashion from predefinedgeometric primitives. If necessary,arbitrarily complex boundary representations could be included asalternatives, but for our purposes the g-prims representations suffice.
The different types of movable joints in the human skeleton canalso be modeled with functions. Conceptually, each function appliesthe required transformations to locate and orient the joint.Joint motions may be restricted to predetermined excursion ranges,one for each of the degrees of freedom of the joint. We use anobject-oriented style of programming in AL to encapsulate the implementationdetails into a joints class. This abstraction allows theinstantiation of joint types to be stated succinctly, which, in turn,simplifies the arrangement of bones and joints into hierarchies.

The arm skeleton

The upper limb of the human body is supported by a complex andintricate skeleton which provides an excellent testbed for developingarticulated models. To simplify interaction, we introduce‘anatomically appropriate’ simplifications to the arm skeleton. Forexample, since the acromioclavicular joint is capable of very littlemotion in itself , we separate the scapula from the arm skeletonand define its motion functionally in terms of avars. Weplace the rooted reference skeleton first, and use nested blockingconstructs to specify the kinematic chain from the sternoclavicularjoint and the clavicle bone down to the wrist joint and the hand skeleton. Low-level motion control is provided by binding avars
to joint angles. High-level motion control is also possible. For
example, by relating a normalized avar clench to the flexion angles
of interphalangeal joints, the fingers of the hand can be clenched
into a fist simply by setting clench equal to one.
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